The present invention relates to refractory mixtures containing magnesia or mixtures of magnesia and chromite formed into compositions or unburnt bricks utilizing boric acid as one primary component of the binder.
In the formation of refractory compositions and unburnt bricks from magnesia and mixtures of magnesia and chromite, a binder is required to give the refractory mixture sufficient strength to retain its shape and configuration and to withstand its environment until the refractory material reaches a temperature sufficient to initiate ceramic bonding of its particulate constitutents. Therefore, the binder for such materials should bond the constitutents rapidly at low temperatures with sufficient strength to withstand the mechanical and thermal environment. Further, the strength of the article should be maintained while the refractory materials are being heated, as for example, when an industrial furnace lined with such materials is heated to pass through intermediate temperatures and then to form the ceramic bond in the lining. Formation of the ceramic bond occurs after passing through intermediate temperatures which are typically called "intermediate zone".
Organic binders such as spent sulphite liquor loose their bonding action at relatively low temperatures and therefore exhibit poor intermediate zone strength, i.e., poor strength at medium temperatures before the formation of the ceramic bond.
Two basic forms of refractory articles are formed from such materials: refractory compositions, which for purposes of this application shall be defined as formable mixtures of refractory compounds, and refractory brick which needs no definition for those familiar with the art. There are difficulties in the use of binders for refractory compositions that are not present for the binders utilized to form bricks from refractory materials. Therefore, there are binders that are suitable for the manufacture of brick that are unsuitable when used in refractory compositions. An example of the different response of binders to the end product formed is shown where sulphuric acid and phosphoric acid demonstrate suitable characteristics of the binder for refractory brick but are unsuitable as a binder for refractory compositions. Similarly, boric acid is not suitable as a binder for refractory compositions since it yields a good intermediate zone strength but practically no bond strength at all at low temperatures.
It is also known to use a mixture of chromic acid or chromites and boric acid as a binder for refractory compositions. The major disadvantage of such materials is that the chromium compounds are a health hazard and the use of chromium compounds as a binder for refractory products is prohibited by regulation in a number of countries.